Geothermal water source heat pump systems are well known, in which a heat pump unit receives ground water and extracts heat energy from the water via a refrigerant in an evaporation/compression/expansion cycle. In the typical HVAC system for a building interior, a geothermal heating system will often include a refrigerant reversing valve and reversible condensers and evaporators, so that the direction of heat flow may be reversed to transfer building interior heat into the groundwater for cooling, depending on the occupant requirements.
There are two basic types of geothermal heating systems using the heat from the ground: closed-loop and open-loop.
In closed-loop systems, the heat exchange fluid is often an antifreeze solution circulated through a closed loop—usually made of plastic tubing—that is buried in the ground or submerged in water (such as an aquifer, well, or pond). A heat exchanger transfers heat between the refrigerant in the heat pump and the antifreeze solution in the closed loop. The loop can be in a horizontal, vertical, or pond/lake configuration.
Open-loop groundwater source systems use well or surface body water as the heat exchange fluid that circulates through the system. Once it has circulated through the system, the water returns to the ground through the well or surface discharge. This option is available wherever there is a supply of relatively clean water. In rural locations where homes and businesses rely on well water, this is often the preferred, less expensive, more efficient system.
Traditional geothermal heating units are relatively large, fixture type installations, intended for whole-house heating and married to the houses that they heat, with permanent installations of underground heat exchange loops (in closed-loop systems), hardwired connections to building electrical power, and permanent plumbing and duct connections to furnace blowers and radiators. In closed-loop systems, the in-ground installation of heat exchange coils and loops can be prohibitively expensive. Conventional open-loop systems may use more water than an existing well can supply.
Additionally, geothermal systems are often used as primary heating, plumbed in series with an existing fossil fuel furnace for additional capacity. Such systems typically require shutting one of the geothermal and fossil systems off when the other is running.